Conventional implementations of copy protection schemes generally work as follows. An encoded video signal is decoded to provide a “clean signal” which is then modified. By modifying certain characteristics of the clean video signal, a specific type of altered video signal is created. When the altered signal is displayed by a display device, it provides high quality video images. However, distortion is also introduced into the altered signal which does not affect its characteristics when displayed over a display device, such as a television set. However, when the same signal is output into another output device, such as a recording device (e.g., a VCR), the modifications made to the signal are sufficient to seriously degrade the output signal to the point where it is unusable, thereby preventing copying of the original video content.
Signals can be rendered unusable by a wide variety of approaches. Some common examples include, but are not limited to, blanking a display screen, introducing so much noise into the signal that it cannot be viewed, introducing flashes of light into the displayed images, reducing the picture resolution to a point where it is unviewable, skewing the color balance to an unwatchable level, introducing picture flaws which are irritating to watch thereby making them unviewable. These techniques, as well as many others known in the art, are used to render the signal completely unusable for viewing or reproduction.
Although the foregoing implementations have the advantage of preventing unauthorized copying of video content, they also have the disadvantage of completely preventing a user from producing personal use copies of his own content. One of the goals of the present invention is to maintain copy protection on existing video content while permitting the creation of “personal use” copies of the same content.
The present invention relates to improvements in digital content copy protection technology. One particular application of this technology is copy protection for digital video content. There exists a need for a method and system for modifying a video signal such that the signal produces a normal, full quality picture on a display device, but also prevents or inhibits the making of high quality reproductions of the original video signal. Secondly, there is a desire by consumers to make personal use copies of the original video signal, even at reduced quality levels. Therefore, there is a need for producing a modified video signal that enables lower quality reproductions of the original video signal to be made. The purpose of these low quality reproductions is to provide a user of video content with the ability to make a personal use copy of the video content. It should be a feature of the personal use copy that, although it is reasonably acceptable for personal viewing, the quality of the copy is such that there is a noticeable decline in signal quality. In particular, the personal use reproduction should be of low enough quality such that it is not salable to the public. Such a method and apparatus will provide the user with the ability to make personal use copies of video content but discourage copying of video content with the intent of committing wholesale piracy of the content.
FIG. 1 graphically depicts a “Horizontal Pixel Resolution Spectrum” which figuratively describes a quality spectrum for video content. As discussed herein, signal quality is synonymous with resolution. Pixel resolution refers to x and y (horizontal and vertical) coordinates with a z coordinate representing the number of bits per pixel (thereby defining color and brightness resolution). As is known in the art, different content formats and video signals can be used to provide different levels of signal resolution and quality. For example, digital television using newer HDTV formats can maintain a typical pixel resolution of 1920 pixels per line. Lower resolution digital television formats can maintain pixel resolutions of 1280 per line or 1440 per line. Typical analog television signals provide a somewhat lower picture quality. For example, resolutions on the order of 720 pixels per line are common. Lower resolutions of 350 to 480 pixels per line are also common. Typical broadcast quality video signal ranges from 400–500 pixels per line and VCR quality signal ranges from 200–400 pixels per line. This spectrum of resolution quality continues to decline until it reaches a level which is essentially worthless and contains no discernible video signal at all.
Moreover, parameters other than pixels per line can be used to define picture quality. Other parameters like sharpness, contrast, brightness, tint, motion continuity, as well as other related parameters, are important to picture quality. Also, content quality is dependent on audio quality. Therefore, a multitude of parameters affect the content quality.